This project transferred form GBB. The former project number was Z01 DK 52013-03GBB. Tay-Sach disease is a group of disorders caused by mutations in the alpha-chain polypeptide of the A form of beta-hexosaminidase, a lysosomal enzyme composed of two chains (alpha, beta). Such lesions result in a spectrum of disease states ranging from severe to mild. Although the disorder is in general rare, both French Canadians living in Eastern Quebec as well as Ashkenazi Jews, have a 10-fold higher gene frequency then the general population for a severe form of the disorder known as "classic" Tay-Sachs disease. We previously found that French Canadian patients lacked a 7.6 kilobase fragment of the alpha-chain gene including the promotor region, exon 1 and part of intron 1, whereas the gene from Ashkenazi patients appeared grossly intact. During the past year we have identified the exact deletion borders in the alpha-chain gene of a French Canadian patient by sequence analysis of the deletion junction in the mutant and corresponding regions of the normal gene. This analysis also demonstrated the presence of similarly oriented Alu sequences at the 5', 3' deletion boundaries suggesting that the deletion may have arisen during homologous recombination from unequal crossing over between Alu sequences. In addition, we have isolated genomic clones from a lambda library constructed with DNA from an Ashkenazi Jewish patient with classic Tay-Sachs disease, that span almost the entire 40 kilobase alpha-chain locus. Twelve of the exons have been sequenced as well as 24 of the 28 splice juction regions. No deviations from the corresponding regions of the normal gene have been found as yet.